While the invention will be described with a particular focus on HPLC, it is not necessarily limited to this application. In HPLC (and also in other applications), pumps are used to generate a flow of a fluid (e.g., a liquid) with a pressure. Such pumps are supposed to convey a flow in the low-pulse to pulse-free range at high pressure. What is used for this purpose are pumps that work based on the displacement principle with pistons having a cyclical effect. For bridging the suction period, pumps with a first and a second head/pair of pistons are used. Both heads can be arranged in parallel with regard to fluidics, so that one piston is always conveying while the other one is suctioning. Such an arrangement is described in U.S. Pat. No. 4,752,385 A, for example. As an alternative, the two heads can also be arranged in series, so that the second piston is conveying when the first one is suctioning, and that the first piston is conveying when the second one is sucking while at the same time being filled by the first piston. Such an arrangement is described in U.S. Pat. No. 4,681,513 A, for example.
One problem to be solved will be described in the following based on the example of a serial double piston pump (while it is noted that the present invention is not limited thereto, but may also comprise other embodiments). FIG. 1 shows a serial double piston pump in a schematic rendering. It comprises a working head 10 inside of which a movable working piston 11 is located. The sealing against the outside is provided by means of a seal 17. The working head has an inlet valve 15 and an outlet valve 16 that are switched in such a manner that the liquid can be sucked in via an inlet connection 14, and that it can be passed on via a connection capillary 24. A pressure sensor 13 can be arranged inside the pump head of the working head in order to determine the pressure in the interior of the working head. The free volume 12 in the interior of the working head can be decreased by the forward displacement of the working piston 11, i.e. in the right direction of the image, or can be increased by a backward movement, i.e. in the left direction of the image. The drive that is necessary for this purpose is not shown in the Figure to providing a clearer illustration. Further, there is a compensation head 20 with a balance piston 21, a seal 27, a free volume 22, and a pressure sensor 23. The compensation head is connected to the connection capillary 24 and to an outlet capillary 30 which provides the conveyed liquid for the HPLC system in a direct manner without any valves. Because the connection capillary 24, the compensation head 20 and the outlet capillary 30 are directly connected to each other, the same pressure is respectively present in these parts, which in the following will be referred to as the system pressure.
The shown components merely serve as examples for explaining the invention. The application to further embodiments will be described below.
The shown pump usually works in a cyclical manner in order to create a continuous flow at the exit. In a first phase of the pump cycle that is referred to as the suction phase, the working piston 11 moves backward and suctions in liquid from the solvent reservoir while the balance piston 21 moves forward, thus maintaining the flow at the pump exit and/or the system pressure. During this procedure, the inlet valve 15 is opened and the outlet valve 16 is closed. The suction phase ends shortly before the balance piston reaches the front end of its duty stroke and thus cannot convey any more liquid.
In a second phase that is referred to as the pre-compression phase, the working piston 11 moves forward to bring the previously suctioned-in liquid to the same high pressure as the one that is present at the pump exit and in the free volume 22 of the compensation head. In the course of this process, the inlet valve 15 closes, with the outlet valve 16 also remaining closed for the time being. This process is referred to as pre-compression, since the liquid has to be regarded as being compressible at the high pressures that are usual in HPLC. During the pre-compression, the balance piston 21 continues to maintain the flow and/or system pressure. The pre-compression phase ends when the pressure in the working head 10 reaches the system pressure, so that the outlet valve 16 opens and both free volumes 12 and 22 are connected to the outlet capillary 30. During the pre-compression phase, the working piston travels the length of the pre-compression path, which depends on the compressibility of the liquid as well as on the pressure inside the compensation head.
In a subsequent third phase (which may be referred to as the conveying phase), the outlet valve 16 is opened, so that the movement of the balance piston 21 as well as of the working piston 11 contributes to the total flow that is provided at the pump exit. In order to avoid an undesired increase of the total flow, the piston speeds have to be accordingly adjusted in such a manner that what results in sum at the pump exit is the desired total flow again. The exact manner in which this occurs depends on the specific technical realization of the pump. In any case, the balance piston 21 must be pulled back in time before the beginning of the next pump cycle or the next suction phase in order to refill the compensation head. In pumps according to the state of the art, this is done either in the third or in an additional fourth phase. When it comes to understanding the invention, it is noted that the flow provided at the pump exit in the third and, where applicable, in the fourth phase depends on the sum of the two piston speeds (with the correct algebraic sign). In the following, the phases following the pre-compression phase are referred to in a generally summarized manner as the conveying phase, independently of any specific technical realization.
The above explanations serve merely for providing an understanding of the invention and are only meant to explain the general working principle of pumps. However, the use of the invention is not limited to the described realization, while it is carried out in such a pump or pump system in some embodiments.
One problem arises as a result of the fact that, during the pre-compression, compression work is applied to the fluid/liquid which is present in the free volume 12 of the working head 10, which leads to this fluid/liquid being heated. This compression work is the greater the higher the pressure and the compressibility of the liquid. Thus, after pre-compression, the pre-compressed liquid inside the working head 10 is warmer than the working head 10 and the working piston 11.
No further compression work is performed following pre-compression, since the pressure in the free volume 12 remains substantially constant after the outlet valve 16 is opened. The previously heated liquid cools off especially at the beginning of the conveying phase due to the contact with the surrounding structural components of the pump, so that its volume and/or pressure changes.
This volume contraction decreases the flow that is provided at that time, which leads to a temporary drop in the provided flow. This is repeated with every pump cycle, and in total manifests as an undesired periodic flow error of the pump. With high pressure gradient pumps (HPG), in which different solvents of multiple individual pumps are mixed, such pulsations additionally manifest as fluctuations of the solvent composition. All these effects lead to a deterioration of the chromatographic reproducibility, which represents an important criterion for the quality of a chromatography system, and they corrupt the signal-to-noise ratio of the detection unit of the chromatography system.